Using ImageJ to Analyze How Lake Mead Has Changed Over Time: Part 1

Lake Mead in recent years. Notice the white coating on the rock. The line is what is known as the "bathtub ring".

Investigation questions:

How has the surface area of Lake Mead changed over time?

How has the water level in Lake Mead changed over time?

Lake Mead was created to store water for agricultural and domestic use, as well as to generate electricity. It has also become an important recreational resource  in 1964 Lake Mead became our country's first National Recreation Area. As the water level drops, all of these uses are affected.

Some experts predict that if Lake Mead falls more than about 50 feet below its January 2010 level, power generation will not be possible.

Lakes are 3-dimensional features  kind of like upside-down mountains. As the water level drops, the volume of the lake decreases dramatically. As of January 2010, the lake was at only 44% of capacity. To get a real sense of how the water level of Lake Mead has changed in recent years, and what this means in terms of the volume of water lost, we need to view the lake from two different perspectives  from above to get a sense of the change in surface area, and from the ground to measure the change in elevation (depth).

Download and Open Images

You're going to use a pair of high resolution Landsat images  one taken in May 2000 and the other in May 2004  to measure changes in the surface area of the lake.

Download the image below by right-clicking (Win) or control-clicking (Mac) on the link and saving the linked file to your Week 2 directory or folder.lake_mead_2000.tif(TIFF 2.9MB Jan27 10)

Download the image below by right-clicking (Win) or control-clicking (Mac) on the link and saving the linked file to your Week 2 directory or folder. If you downloaded this image in the Getting to Know Measuring in ImageJ section, you won't need to download it again. lake_mead_2004.tif(TIFF 2.9MB Jan27 10)

Make a Visual Comparison

These true color satellite images show the exact same view of Lake Mead, four years apart. Slight color differences between the two images are mostly due to differences in atmospheric conditions on the two dates.

To make an initial side-by-side comparison of the two images, choose Window > Tile.

Another way to compare the images is to display both images in the same window and flip back and forth between them. In ImageJ a window containing multiple images is called a stack, and the individual images are called slices. Stacking images is a useful way to look at images that show change over time. In addition to flipping through images, you can also animate them. (You'll learn a lot more about stacks in Week 3!)

Create a stack To combine the two images into a stack, choose Image > Stacks > Images to Stack.

Animate the stack Use the right and left arrow keys or the scroll bar at the bottom of the stack to flip back and forth between the two images. Note: Some color differences of the lake itself are due to actual changes in the lake, others are due to atmospheric effects.

What are the main differences you see between the images of the lake on the two dates? Look for specific features in and around the lake that change dramatically. Think about how these changes affect both people and wildlife.

The lake is noticeably smaller and you can see the "bathtub ring". It is now much more difficult for people to launch boats and for fish to survive in the shallower water. Some islands are now connected to the shore, making once-safe habitat more accessible to predators. Elsewhere, new islands have appeared, creating new habitat for some animals.

Set the Scale

Now you'll set the scale on these images using the distance you measured between the two islands. One reason for stacking the two images is that setting the scale for one image in a stack automatically sets the scale for the others.

Use the arrow keys on your keyboard to flip to slice 2 of the stack. This slice is labeled 2/2 in the image window status bar.

Use the Straight line selection tool
to select the distance between the two islands.

Choose Analyze > Set Scale.... Enter 25.4 for the Known Distance, km for the Units of Length, and click OK.

Measure Practice measuring a few distances and areas on the images and see if they look reasonable.

Add a scale bar Flip to the first slice (the 2000 image), choose Analyze > Tools > Scale Bar... and add a scale bar to the image. Flip to the second slice and add an identical scale bar to the 2004 image.